We´re using http://handsontable.com/ in our project and are very happy with it so far.

There is for example http://handsontable.com/

Incidentally, I have been playing around with the concept myself (client work) and it turns out you can hack a semi-working solution in just a handful lines of js:

http://soquel.github.io/spreadsheet-in-js/

I know a dirty Spreedsheet for browser. I first seen it in IRC, iirc. I was a greedy copycat to paste it into my site, for the case paste server goes down.

http://kephra.de/copycat/zarasheet/

I kept the references. Both the original fiddle and the authors website are linked from my page.